Electromagnetic transducers convert motion or vibrations into energy. They generally include, as shown in the sectional view of FIG. 1, a permanent magnet adapted to slide into a solenoid. The equivalent circuit of electromagnetic transducers is shown in FIG. 2.
Commonly, it is understood that the permanent magnet slides through the solenoid with a sinusoidal oscillation motion, thus generating a sinusoidal voltage V on the solenoid in an open circuit condition. The voltage generated by an electromagnetic transducer should be modeled as a sinusoidal voltage with random amplitude and frequency. For sake of simplicity, in the ensuing description it will be assumed that the electromagnetic transducer generates a sinusoidal voltage with constant amplitude and frequency in open circuit conditions, though the same observations hold mutatis mutandis even if a sinusoidal voltage of random amplitude and frequency is considered.
The electric energy generated by electromagnetic transducers is scavenged according to the electric scheme of FIG. 3, using a rectifying bridge of the generated voltage and a common step-up converter charging an output capacitor Cout. This approach is widely used, but its efficiency is relatively poor.